1. Field of the Invention
The present invention generally relates to a motor-driven power steering system of a motor vehicle or automobile which is adapted for assisting a driver in manipulating a steering wheel or handle by making use of an assist torque generated by a reversible motor which is operatively coupled to the steering system. In particular, the invention is concerned with a control apparatus for the motor-driven power steering system which allows smooth transition of steering operation from a motor-aided mode to a manual mode when a fault or abnormality occurs in the assist torque generating motor or other parts. The invention is also concerned with an apparatus for controlling an electromagnetic clutch through which an assist torque generated by the motor is transmitted to the steering system such that a coupling effort of the clutch decreases continuously to a level allowing the manual operation of the steering wheel upon occurrence of abnormality in the motor. The combination of the motor-driven power steering system and the control apparatus may also be referred to as the motor-driven power steering control system.
2. Description of the Related Art
For better understanding of the present invention, description will first be directed to a motor-driven power steering control system known heretofore. FIG. 6 is a schematic diagram showing a general arrangement of a conventional motor-driven power steering control system described in, for example, Japanese Unexamined Patent Application Publication No. 255273/1987 (JP-A-62-255273) or Japanese Unexamined Patent Application Publication No. 215461/1988 (JP-A-63-215461).
Referring to the figure, a steering mechanism is comprised of a steering wheel 1 to which a steering torque is applied by a driver of the motor vehicle (not shown), a steering shaft assembly 2 for transmission of the steering torque, universal joints 4 provided for operatively coupling the individual shafts of the steering shaft assembly 2, a pinion shaft 5 provided at a bottom end of the steering shaft assembly 2, and a rack 6 disposed to mesh with the pinion shaft 5. A steering torque sensor 3 is provided in association with the steering shaft assembly 2 for detecting a steering torque T transmitted there through. Further, a steering speed sensor 7 is provided for detecting a steering speed A at which the steering wheel 1 is manipulated by the driver.
For the purpose of controlling the steering mechanism, there is provided a control apparatus 9 which includes a microcomputer for controlling an electric motor (or reversible motor) 13 and others in a manner which will be elucidated later on. A vehicle speed sensor 10 is provided for inputting a vehicle speed V to the control apparatus 9. A power source for the control apparatus 9 and the electric motor 13 is constituted by an onboard battery 11, wherein a key switch 12 is interposed between the control apparatus 9 and the battery 11. The power supplied to the electric motor 13 is controlled by the control apparatus 9. To this end, an electromagnetic clutch 14 is disposed at the output side of the electric motor 13 and adapted to be controlled by the control apparatus 9. The electromagnetic clutch 14 has an output shaft coupled operatively to a worm shaft 15 which constitutes a part of a reduction gear unit. Provided so as to mesh with the worm shaft 15 is a worm wheel 16 meshing with a pinion shaft 18 which in turn meshes with the rack 6.
As is apparent from the above description, the electric motor 13 is operatively coupled to the steering mechanism via the electromagnetic clutch 14 for generating a steering assist torque which is applied to the steering mechanism.
FIG. 7 is a functional block diagram showing a configuration of the control apparatus 9. As can be seen in the figure, the control apparatus 9 is comprised of a current calculating/driving means 91 for calculating a motor current command value I.sub.0 on the basis of the steering torque T and the vehicle speed V to thereby drive the electric motor 13 on the basis of the motor current command value I.sub.0, a clutch control means 92 for changing over a clutch control signal C for the electromagnetic clutch 14 from an ON-level to an OFF-level when the vehicle speed V attained a predetermined reference vehicle speed V.sub.0 (e.g. 50 km/h), and a motor current detecting means 93 which serves for detecting an actual motor current I of the motor 13, wherein the signal indicative of the actual motor current I is supplied to the current calculating/driving means 91.
With the arrangement of the control apparatus 9 described above, the control apparatus 9 controls a voltage applied to the electric motor 13 so that a current corresponding to the motor current command value I.sub.0 flows through the electric motor 13 by monitoring the actual motor current I as detected and fed back. On the other hand, the clutch control means 92 is adapted for interrupt (i.e., turn off) the clutch control signal C when occurrence of a fault in a low-speed operation range of the motor vehicle is determined on the basis of detection signals available from outputs of the relevant sensors (not shown) which the motor vehicle is equipped with. Incidentally, the motor current detecting means 93 may be constituted by, for example, a resistor having one end grounded.
FIG. 8 is a view for illustrating changes in the clutch control signal C and the motor current command value I.sub.0. As is shown at (a) in FIG. 8, the clutch control signal C is at the ON-level (indicating the clutch current on the order of e.g. 1 ampere) when the vehicle speed V is lower than the predetermined reference vehicle speed V.sub.0 and changed over to the OFF-level (i.e., the level corresponding to zero clutch current) when the vehicle speed V is higher than the predetermined reference vehicle speed V.sub.0 inclusive thereof. Further, as shown at (b) in FIG. 8, the motor current command value I.sub.0 is so changed as to increase when the vehicle speed V decreases or as the steering torque T increases so long as the vehicle speed V is lower than the predetermined reference vehicle speed V.sub.0 and is interrupted (i.e., set to zero level) when the vehicle speed V is higher than the predetermined reference vehicle speed V.sub.0 inclusive thereof. In general, the steering wheel 1 is heavy to steer (i.e., the steering torque T as required is large) when the motor vehicle is driven at a low speed, while the steering wheel 1 is easy to steer (i.e., the steering torque T as demanded is low) when the motor vehicle is running at a high speed, even though it also depends on the road conditions more or less*. Accordingly, the steering assist torque is increased as the vehicle speed V becomes lower.
Next, referring to FIG. 8, description will turn to operation of the motor-driven power steering control system shown in FIGS. 6 and 7.
In the course of a low-speed running of the motor vehicle in which the vehicle speed V is lower than the predetermined reference vehicle speed V.sub.0, the clutch control means 92 incorporated in the control apparatus 9 maintains the clutch control signal C at the ON-level to allow the electromagnetic clutch 14 to be operatively coupled to the electric motor 13. Further, the current calculating/driving means 91 generates a signal indicating the motor current command value I.sub.0 determined on the basis of the vehicle speed V outputted from the vehicle speed sensor 10 and the steering torque T derived from the steering torque sensor 3 to thereby drive the electric motor 13 so that a steering assist torque as required is generated by the electric motor 13.
More specifically, because the steering torque T of greater magnitude is required as the vehicle speed V is lower, the motor current command value I.sub.0 which determines ultimately the steering assist torque is set at a correspondingly large value. Further, when a greater steering torque is required in dependence on the road conditions, the motor current command I.sub.0 is set to a correspondingly large value.
In this way, magnitude of the steering torque T which the driver is demanded to apply to the steering wheel can be maintained substantially or approximately constant regardless of the running conditions of the motor vehicle.
Incidentally, when occurrence of a fault in the system is detected in the course of driving the motor vehicle, the clutch control means 92 interrupts the clutch control signal C to thereby disconnect the electromagnetic clutch 14 from the electric motor 13 in order to ensure safety for the steering operation. In this conjunction, occurrence of fault or abnormality can be decided when the motor current command I.sub.0 or the actual motor current I indicates an abnormal value and/or when the steering aid operation for the steering wheel 1 remains ineffective over an extended time span.
On the other hand, in the high-speed driving range where the vehicle speed V is higher than the predetermined reference vehicle speed V.sub.0, the current calculating/driving means 91 sets the motor current command value I.sub.0 to zero while the control apparatus 9 interrupts the clutch control signal C to thereby disconnect the electromagnetic clutch 14 from the electric motor 13. In that case, since no voltage is applied to the electric motor 13, the latter remains inactive. Further, occurrence of the system fault during the high-speed operation will involve no obstacle to the manipulation of the steering wheel. When the vehicle speed V becomes lower than the predetermined reference vehicle speed V.sub.0, the clutch control signal C is again changed over to the ON-level, as a result of which the electromagnetic clutch 14 is operatively coupled to the electric motor 13 for thereby effectuate the control of the actual motor current I in dependence on the vehicle speed V and the steering torque T, as mentioned previously.
In contrast, when abnormality occurs in the control apparatus 9, an erroneous motor current command value I.sub.0 may be generated to drive the electric motor 13 unwantedly, whereby the steering wheel 1 is forcibly rotated. Further, occurrence of a mechanical fault in the electric motor 13 would render it difficult for the driver to manipulate the steering wheel 1, incurring a serious situation that the steering can no more be facilitated. Although these inconveniences can certainly be tolerated in the low-speed operation range, they should never take place in the high-speed operation, because, if other wise, the safety for the steering operation is remarkably lowered.
Under the circumstances, by adopting such an arrangement that when the vehicle speed V becomes higher than the predetermined reference vehicle speed V.sub.0, the clutch control signal C is changed over to the OFF-level (zero), the safety feature is ensured for the steering operation.
With the conventional motor-driven power steering control system of the structure described above, the electromagnetic clutch 14 is electrically energized in the lower-speed operation so long .as no system fault takes place, while in the high-speed operation, the electromagnetic clutch 14 is disconnected from the electric motor 13 regardless of whether the system fault occurs or not. Thus, every time the vehicle speed V exceeds the predetermined reference vehicle speed V.sub.0, the electromagnetic clutch 14 is disconnected from the motor 13, which is accompanied with generation of remarkably large noise, which is of course undesirable from the standpoint of comfortableness in driving the car. Besides, frequent on/off operation of the electromagnetic clutch 14 promotes degradation of durability and shortens the use life. Additionally, the on/off operation of the electromagnetic clutch 14 may be transmitted as shock to the driver who is steering by gripping the steering wheel 1.